1. Field of the Invention
This invention relates to the design and fabrication of air turbine dental handpieces ("dental drills"), and more particularly to a high speed air turbine dental handpiece having reduced noise levels.
2. Description of the Prior Art
Many dentists use a high speed air-driven impulse turbine dental handpiece in their practice to drill out cavities, to prepare crowns, and to drill holes for cap pinning. It is unfortunate that the prior art air turbine devices characteristically produce high noise levels that are systematically destroying the hearing of many dentists. The first high speed dental handpiece using an air-driven impulse turbine with jewel bearings was patented in 1963 by Dr. Jacob A. Saffir (U.S. Pat. No. 3,071,861), a dentist practicing in Los Angeles, California. Many of the prior art dental handpiece designs are modified versions of Dr. Saffir's design. There has been little or no improvement on the subject of drill noise reduction since 1963.
The conventional high speed dental handpieces manufactured today typically have the following common design features: (a) they use a 40 psig air supply; (b) they have an impulse turbine located in the drill head; (c) they use standard instrument ball bearings; (d) they have an uncontrolled air exhaust from the drill head; and, (e) they have a water mist system for burr cooling.
In the year 1983 there were approximately 35,000 licensed dentists in the State of California alone. Like other dentists worldwide, virtually all rely upon the pneumatic handpieces now on the commercial market. Nonetheless there is mounting evidence and a growing conviction among audiologists and dental personnel that the high noise levels produced by these conventional dental handpieces induce loss of hearing quite apart from damage due to natural causes.
In general, noise is undesirable because it may: (a) damage hearing, either temporarily or permanently; (b) mask other audible warning signals which ought to be heard, leading to confusion and causing accidents; (c) distract persons from concentration on important tasks and so cause mistakes and inefficient production; and, (d) hinder speech communication.
An important source of noise produced by conventional high speed dental handpieces is turbulence in the working fluid (air). Mechanical vibration of a surface is also a source of noise.
Pulsations and gas flow are generated by tangential flow turbo machinery because of their mode of operation. The noise frequency of pulsating flow is a multiple of the shaft speed and depends on the number of rotor blades. Pulsating flow produces noise as discrete frequencies corresponding to the fundamental frequency of the pulsations and its harmonics. This appears to be the primary noise source in the conventional dental handpiece although noise occurring over a band of frequencies, such as that produced by high speed jets, may also be a significant contributor.
Actual acoustic testing has shown that conventional high speed dental handpieces have not achieved their advertised noise levels of "less than 70 dBA". In fact, noise levels ranging from 76 dBA to 102 dBA at 4000 hz. have been measured.
Dental assistants also suffer the same plight as the dentist, i.e., irritability, fatigue, and gradual loss of hearing. Patients are not exposed for a long enough period of time to the high pitched sound to suffer hearing loss. However, patients experience an undeniable psychological effect due to the high pitched noise that can be described as negative. Patients are frequently afraid of the noise of the conventional high speed dental handpieces.